Biotite

Biotite or black mica (magnesium iron mica) is a common phyllosilicate. It is since 1999 under the decision of the International Mineralogical Association (IMA ) is no longer as an independent mineral, but as a mixed crystal of Annite - phlogopite mixed batch. Biotite belongs to the " white mica " muscovite mica of the most common types and is eponymous for the Biotitreihe the mica group.

The mineral crystallized flake- shaped in the monoclinic crystal system with the general formula K (Mg, Fe2 , Mn2 ) 3 [ (OH, F) 2 | (Al, Fe3 , Ti3 ) Si3O10 ]. It develops translucent to opaque, very flexible and partially pseudo-hexagonal crystals with metallic pearly luster, which can be split into very fine flakes.

It was named (1774-1862), who first recognized the visual diversity of mica after the French physicist Jean -Baptiste Biot.

Education and Locations

Biotite is rock-forming in many igneous rocks in front, for example, in granites. From an ascending molten rock just before it crystallizes feldspar, quartz and pyroxene, when the magma cools to about 800 to 700 ° C ( phase of main crystallization ). Is widespread biotite in metamorphic rocks such as gneiss and mica schist. In sand and sandstone, he finds himself in the form of small, shiny gold leaves.

The layer structure in many metamorphic rocks (eg gneiss) goes back to the different temperature behavior of the dark mica in the segregation of the rock at greater depths. Biotite and other layer silicates tend to crystallize under hot pressure load new lateral, while quartz and feldspars remain rather grainy. The foliation surfaces correspond to the direction of maximum tectonic shear forces.

Like the muscovite biotite forms in pegmatites large panels. Light and dark mica can grow together, the cleavage planes pass through both mica without interruption. In addition, biotite forms with many other minerals assemblages, so among other things, with amphibole, andalusite, cordierite, grenades, potassium feldspar, nepheline, plagioclase, pyroxene, quartz, spinel

By chemical weathering of the mineral is easily attacked, loses luster and elasticity and becomes brittle. In the final stage of decomposition to iron hydroxides and various clay minerals ( hydrobiotite, vermiculite), fool's gold or chlorites form. If biotite deposited as fine flakes in the water on sand, so he makes changes there by (eg oxidation). The color takes a golden brown tone, hence the name fool's gold. Is used economically biotite only as an additive in cosmetics and as a filler (together with muscovite ).

So far, biotite was detected at about 4,600 localities (as of 2010). Larger sheet crystals can be found inter alia in the area around the Laacher See in Germany, in France Bessnes, Ontario in Canada, Evje in Norway, as well as in the Uluguru Mountains in Tanzania. Countries with many sites (5 or more), including Argentina, Australia, Bolivia, Brazil, Bulgaria, Chile, China, Germany, Ecuador, Finland, France, India, Indonesia, Italy, Japan, Kazakhstan, Madagascar, Mexico Mongolia, Namibia, North Korea, Norway, Austria, Papua New Guinea, Peru, Philippines, Poland, Paraguay, Portugal, Romania, Russia, Sweden, Switzerland, Slovakia, South Africa, South Korea, Spain, Czech Republic, Ukraine, Hungary, the United Kingdom ( UK ) and the United States of America (USA).

Petrology

Biotite occurs in almost all metasediments. In the amphibolites with the mineral amphibole and plagioclase in equilibrium while in the eclogites a degradation product of phengite is.

Use

For the use of rocks as building material biotite turns out to be unfavorable, because it 's rapid penetration of weathering solutions permits and therefore also frequently forms rust stains on the surface ( eg at curbs of granite ) due to its cleavage. As biotite is relatively soft, it is in pure form as a building material without use. However, it is used in the cosmetic industry, together with the muscovite to give a cosmetic mica minerals typical for all the glittering effect ( see also using mica ).